In the process of drilling a well, solids, fluids, and gases are brought to the surface by the drilling fluid (mud) and are collected and analyzed. Drilling, drill-in, and completion fluids are typically analyzed several times a day for several key-chemical properties to ensure the technical, environmental and safe performance of the drilling fluid. Examples of properties tested in drilling, completion, and production fluids are: alkalinity, chloride content, polymer concentration, density, hardness, iron content, methylene blue test, pH, potassium content, silicate concentration, sulfide concentration, salinity, shale inhibitor content, hydrate inhibitor content, asphaltene precipitation inhibitor content, heavy metal concentration, and compatibility of formation fluid with the completion fluid.
Typically, these tests are one or two daily measurements of the fluid with simplistic, yet relatively time-consuming analytical tests. On occasion a more detailed analysis is performed. Considering the complexity of the fluid systems, the high cost of drilling and completing a well, and the danger of production losses, such tests are not entirely satisfactory analytical tools. It would be desirable if there were a method available to analyze chemical properties of drilling mud, completion fluid or production fluid that was faster, more flexible, and provided a higher reproducibility of data.
Chemical sensor techniques and especially micro-electrical machines (MEMs) that measure biochemical properties are being developed in the biomedical industry. Some of the measurement techniques incorporated in these measurement devices may include, for example, optical spectroscopy (UV, visible, infrared, and near infrared), potentiometer, calorimeter, selective chemical sensors, optical rotation, diffusion, and chromatography.
The measurement principles for chemical sensors and chemical sensing devices for liquids and gases are in some cases similar, but the applications are generally very different due to the great differences in complexity and variability of the target species. Thus, gas measurements involve chemically very simple species that typically do not interact significantly with each other, either in the gas phase or in the analytical system. In contrast, liquids, especially those associated with drilling-, completing and producing wells, contain complex mixtures of solids, ionic species, surface-active agents of a variety of types, hydrocarbons, water, and polymers. Those knowledgeable in the art will recognize that not all of the techniques mentioned above are appropriate for both gases and liquids and that, even when the principles are similar, application of the techniques is very different.
U.S. Pat. No. 5,306,909 discloses a method of analyzing drilling muds that involves the use of reflectance infrared spectroscopy. The method utilizes standards of mud components for calibration and provides estimates of the concentrations of various polymers in water- and oil-based muds, water, water chemical activity, and quartz and other minerals in oil-based muds within accuracy of 10-20%. The authors claim the ability to estimate values for a variety of mud properties through use of calibration with muds for which these properties are known. Important points to note are that the method relies on a calibration set of well-characterized materials, which may or may not correspond to materials in field use, and has very limited accuracy for the mineralogy estimates, with no indication of the accuracy of the other estimates. In addition, while the method is claimed to require no sample preparation, the importance of scattering limits the particle size distribution in the samples to be analyzed. Thus, it is asserted that adaptation to an on-line application would be limited.
U.S. Pat. No. 6,176,323 discloses a method of analyzing the chemistry of drilling fluids, as well as the concentrations of tracers in these fluids. Specifically, the patent claims the ability to measure: “(a) presence of a hydrocarbon of interest in the drilling fluid, (b) presence of water in the drilling fluid, (c) amount of solids in the drilling fluid, (d) density of the drilling fluid, (e) composition of the drilling fluid downhole, (f) pH of the drilling fluid, and (g) presence of H2S in the drilling fluid”. These measurements are obtained using optical spectroscopy alone, total reflectance alone, and optical spectroscopy combined with sol/gel technology to provide a medium for reactions of chemicals in the mud with chemicals in the glass to provide color centers that can be detected optically. The chemicals in the mud can be added as part of the mud program or can be present as the result of influx from the formations being drilled.
The total reflectance system requires the use of two separate detectors, one for the incoming mud and one for mud that has passed through the drill bit, with subtraction of the former spectrum (or intensity at a single wavelength) from the latter spectrum (or intensity at a single wavelength), the difference being the spectrum (or intensity) of the species of interest. Note that the mud that has passed through the drill bit (the mud in the annulus) contains drill solids, particles of various sizes acquired as the result of drilling, ranging in diameter from a micron or two to centimeters. Unfortunately, total reflectance is well known to those knowledgeable in the art to be notoriously dependent on particle size in turbid fluids and to be difficult to quantify in the laboratory. Thus, a subtraction operation applied to two profiles that are not quantitative is highly unlikely to produce useful results.
The sol/gel approach is feasible for single-use measurements if the target species can be reacted in such a way as to produce a species with color that can be detected by the optical system. Of the mud contaminants listed in the various claims, the one most likely to be detected by a color-generating reaction is H2S; for this reason, the remainder of the discussion will focus on that material. The method relies on diffusion of the target analyte through a porous glass medium to encounter a chemical species designed to react with it to produce a colored product. The system requires water-based mud for optimum reactivity since the glass is water-wet, and the reactions take place in an aqueous medium. Thus, this approach is highly unlikely to be useful with synthetic-/oil-based muds, which have hydrocarbons as the external phase. For water-based muds, the pH is typically at around 9 and is often higher, with the result that an incursion of a few, or even a few hundred, ppm of H2S is not considered a problem; detection at the surface is adequate. Operators concerned about the possibility of H2S typically raise the pH of their muds, add a buffer to provide extra pH protection, and/or add a sulfide scavenger, of which several are commercially available.
A key element of the sol/gel approach is that once a unit has reacted to produce a species detectable by optical spectroscopy, it cannot be re-used. Thus, the method cannot be used to evaluate mud chemistry downhole on a routine basis.
The authors of '323 claim that the detection methods include the use of “visible light, infrared, near infrared, ultraviolet, radio frequency, electromagnetic energy, and nuclear energy”, however all are not demonstrated in the patent. Electromagnetic energy encompasses all of the preceding types in this list, and the patent provides no description of the use of nuclear energy. The authors only provide a description of a non-radioactive tagging atom, deuterium.
The use of micro-systems is known in art relating to, for example, biology, medicine, and explosives. Sandia National Laboratories (a DOE organization) has disclosed the use of micro-systems to detect a variety of products. This organization has investigated the use of micro-scale gas chromatography coupled with surface acoustic wave sensors to detect explosives, among other materials. Furthermore, they have developed a hand-held device involving micro-scale chromatography, with the specific application of the detection of explosives. Sandia has also developed two methods of chemical detection for application to explosives and weapons of mass destruction. An underwater unit, for explosives, utilizes an ion mobility spectrometer in identifying the chemical signatures of the materials. A vapor unit relies on acoustic wave sensors developed for specific compounds. It is important to note that these systems have only been applied to explosives, and that single-use units are not appropriate for flowing systems in which repeated measurements must be made. The concept of analyzing aerosol particles is patented by Sandia in U.S. Pat. No. 6,386,015: “Apparatus to collect, classify, concentrate, and characterize gas-borne particles.”
Investigators at the University of Washington, Seattle, are developing continuously operating methods of measuring the concentrations of a variety of analytes, but the primary field of application is medical. See, for example, U.S. Pat. Nos. 5,716,852; 5,948,684; 5,972,710; 5,974,867; 5,932,100; 6,134,950 and 6,387,290.
U.S. Pat. No. 5,910,286 discloses a chemical sensor that relies on a “molecular fingerprint”, in which a cavity created in a crosslinked polymer is exactly the size and shape of the target analyte. The focus of this approach is a single-use detection of a single chemical species. No specific application is described.
U.S. Pat. No. 5,984,023 discloses downhole in-situ measurement of physical or chemical properties of cores as they are being taken, including porosity, bulk density, mineralogy, and fluid saturation. The major focus is on rock properties, and the limited fluid analysis aspects of the patent are so general as not to be applicable to analysis of mud, completion fluid, or production fluid analysis.
U.S. Pat. No. 6,023,540 builds upon two earlier patents (U.S. Pat. Nos. 5,244,636 and 5,250,264) in describing the development of fiber optic bundles with multiple arrays of microspheres coated with specific materials that react with the target analyte to provide a unique signature. While no specific application or type of application was described for this tool, the approach is single-use and is not feasible for use in a flowing fluid.
U.S. Pat. No. 6,070,450 describes a gas sensor capable of detecting methane and carbon monoxide in a single device using a layer structure, in which the surface of a sensor for “city gas” (methane and the like) is overlain by a sensor for incompletely burned gases such as carbon monoxide. This device is designed for use as part of an alarm system and is single-use.
U.S. Pat. No. 5,822,473, which builds upon U.S. Pat. No. 5,760,479, describes the development of an integrated microchip chemical sensor designed to detect the presence of a single compound in a single-use application. Again, these single use systems would be unfit for flowing systems in which repeated measurements must be made.
U.S. Pat. Nos. 5, 610,708 and 5, 502,560 describe mechanics of measurement for a micro-scale grating light reflection spectroscopy probe for use as a process monitor. The application specifically described is particle size discrimination of dendrimeric particles in water solution.
The Center for Process Analytical Chemistry (CPAC) at the University of Washington (UW) sponsors NeSSI, the New Sampling/Sensor Initiative, to develop new miniature sampling systems based on semiconductor standards adapted to use in providing real-time feedback on the concentrations of multiple chemical species in process chemistry, but no patents regarding microfluidic testing or devices have been granted. The objective of the work is the development of methods of handling the information rather than the development of new sensors.
Other research abstracts from CPAC propose: 1) vapochromic detection and identification of volatile organic compounds applicable to a variety of simple compounds in a single-use approach; 2) surface plasmon resonance sensors, with proposed applications for bioanalytes; 3) Raman spectroscopy for process monitoring and materials characterization (Historically this approach has been limited to white solids or colorless solutions since the intense power of the laser is absorbed by darker-colored materials, resulting in rapid temperature increases. Application to drilling fluids, for example, would be limited on this account. And application to, for example, drilling fluids would be limited due to the complexity of the spectra and the insensitivity to metal ions); 4) Process liquid chromatography and sampling on a micro-scale are under development; and 5) The concept of a “Lab-on-a-Valve” for the monitoring of fermentation. No patents are known to have issued on any of these concepts in development.
APS Technology, Hellertown, Pa., has a DOE contract for a three year project for the development of a downhole sensor for production fluids, however it only contemplates a very gross evaluation of water, gas, and oil.
Microfluidics is a term for chemical analysis executed in microsized channels etched on a glass plate. An early patent for a microfluidic device is U.S. 5, 376,252, assigned to Pharmacia Biosensor. Microfluidic sensors and devices have been developed primarily for medical and biotechnology applications. Analyses such as, for example, DNA and RNA characterization have been carried out on small chips. For example, U.S. Pat. Nos. 6,358,387; 6,306,659; 6,274,337; 6,267,858; 6,150,180; 6,150,119; 6,132,685; 6,046,056; and 5,942,443, assigned to Caliper Technologies, Inc., disclose microfluidic devices and methods that are useful for performing high throughput screening assays. For an overview of microfluidics see Whitesides, G. M., Strock, A. D., [2001] “Flexible Methods for Microfluidics,” Physics Today, 54 (6), 42-48.
There is a need in the art for a convenient method of continuous or intermittent measurement and analysis of drilling fluid chemicals. It would be particularly desirable if the method required only small amounts of sample and reagents and provided reliable, reproducible results. Deficiencies in the drilling fluid or the presence of influxes could be detected in real-time, potential well control or hazardous situations could be avoided, appropriate treatment could be applied, costly mud-related delays could be averted, and expensive production shut downs minimized. Such a system could more efficiently address drill fluid chemistry problems relating to drilling fluid flocculation and chemical imbalances and hazardous influxes of H2S, CO2, and CH4. In addition, the method could also provide valuable measurements of hydrocarbon gases, noxious gases, crude oil, water, tracers, and inhibitor (scale and asphaltene deposition, hydrate formation) concentrations.